A television signal may include auxiliary information in addition to video and audio program information. For example, data pertaining to digital data services such as closed captioning (CC), EXtended Data Services (XDS), and StarSight.RTM. may be included in National Television Standards Committee (NTSC) television signal in the United States. The digital data for such services is encoded in various portions of the vertical blanking interval (VBI) of an NTSC television signal.
Specifically, closed captioning data is encoded in the vertical blanking interval of line 21 of a television signal. Each line 21 vertical blanking interval that includes captioning data has two bytes of data encoded in the latter half of the line interval. The data is processed to produce text representing the audio content of a television program in a portion of the television display. Although captioning was originally developed to aid the hearing impaired, captioning also provides benefits to non-hearing impaired users. For example, by enabling both captioning and audio muting, a user can visually enjoy both the video and the audio program portions of a television program without disturbing others. United States law requires closed caption decoders in all television receivers having displays larger than 13 inches (FCC Report & Order, FCC 91-119). As a result, most video programming (including video tape) now includes captioning data.
EXtended Data Services (XDS) data is encoded in the same format as closed captioning data and shares line 21 of field 1 with the closed captioning data, i.e., each video frame contains two fields of video information denoted as field 1 and field 2. As specified in ANSI/EIA 608, XDS data provides information such as program rating/content (e.g., can be used to provide the so-called V-Chip function for restricting television viewing based on program content), program description, program title, program start time, elapsed time, network name, station identification and future program information.
StarSight.RTM. data provides information similar to XDS and is encoded in the same format. However, StarSight.RTM. data provides information regarding all channels rather than just the present channel. In addition, StarSight.RTM. data can be included in the VBI of one or more of lines 10 to 18. StarSight.RTM. data is decoded to produce a program guide display similar to program guides provided in print media. Because StarSight.RTM. data includes data for all channels, receiving StarSight.RTM. data for a complete program guide display can require up to six hours.
A decoder is required for extracting any of the foregoing types of auxiliary data. One component of the decoder is a data slicer that converts the analog television signal into digital data during the auxiliary data interval. Because closed captioning, EXtended Data Services and StarSight.RTM. all use similar data encoding formats, one data decoder could be shared for recovering data for all three data types. However, the data decoder in a conventional television receiver is hardwired to the main video source for decoding captioning and XDS information pertaining to the main video source.
Hardwiring the data decoder to the main video source limits the flexibility of the decoder. For example, auxiliary information that is associated with an auxiliary image in a multi-image display cannot be decoded. One specific example is a picture-in-picture (PIP) system in which auxiliary information associated with the small picture (pix) that is inset into the main picture cannot be decoded. Another example is a picture-outside-picture (POP) system. As a result, caption information for the auxiliary image cannot be displayed. Also, XDS-related features, such as restricting what can be displayed based on program content (, i.e., V-chip control), cannot be implemented for the small picture. In addition, because auxiliary data such as StarSight.RTM. data may be present on a channel other than that being viewed via the main picture source, hardwiring an auxiliary data decoder to the main signal source may preclude accumulating StarSight.RTM. data during normal viewing.
An approach to improving the flexibility of the decoder is to allow video sources other than main video to be coupled to the input of the data decoder, e.g., by adding a selector switch. Adding a second tuner further improves the flexibility of the auxiliary data recovery system. As an example, certain television receivers produced by Sharp, e.g., models 31HX1200 and 35HX1200, include two tuners and a data decoder with an input selector switch. These receivers support viewing captions for either the main picture or the small picture.